I am trying to replicate a VM from our primary site to a standby site. The VM has a single 30GB disk. The VM hardware version is Version 7. Both sides have ESX servers running ESX 4.0 Advanced (vStorage API is listed as one of the licensed features). The sites are connected via site to site VPN tunnel (20MB upstream FiOS on one side and 100MB ethernet to the internet on the other side).

I can successfully run replication jobs of this VM from one site to the other, WITHOUT changed block tracking (it processes the full size of the VM every time it runs). When enabling changed block tracking on the replication job, it allows me to do so, but when I run the job and look at the HTML realtime statistics it says the replication mode is "Network NBD WITHOUT changed block tracking."

Any ideas why? Is there something I need to do in ESX to make this work?

No, you don't need to do anything additional to your ESX host to make it work.

Basically, for Change Block Tracking you only need ESX 4 or later host, and hardware version 7 VM.

You can always check if you have this line presented ctkEnabled (value = true), under VM Edit Settings -> Options -> Advanced -> Configuration Parameters. If it is present, CTK has been already enabled on this VM by Veeam Backup. And in this case, along with the line ctkEnabled = true, there should be other lines for the VMDKs as well. For ex : scsi0:0.ctkEnabled = true; scsi0:1.ctkEnables = true and so on.

Also please check if you have snapshots presented, because Change Block Tracking feature cannot be enabled if you have one or more snapshots present on VM. You will be able to create snapshots later, once Change Block Tracking has been enabled.

But can you tell me if you see any warnings during the job run? It'll be great if you could send us the logs from Help->Support Information to support@veeam.com, so we could see why change block tracking feature is not working for you.

Correct, everything is fully automated. Veeam Backup will either enable change tracking automatically for you, or will log warning in the session's log on why it could not do this (e.g. incorrect VM hardware version, snapshots present and so on).

Yes, this specific one is just more rare occurence. If I recall correctly, this one will appear if some disks were added/removed to VM after changed block tracking was first enabled, or something like this.

Basically there are no more special tricks to make CBT work, so to get a further assistance I suggest contacting our technical team. Also I would appreciate if you could update this topic with a resolution once you have it resolved.